Req is an easy-to-use, type-safe, expandable, high-level HTTP client library
that just works without any fooling around.

What does the phrase “easy-to-use” mean? It means that the library is
designed to be beginner-friendly so it’s simple to add to your monad stack,
intuitive to work with, well-documented, and does not get in your way. Doing
HTTP requests is a common task and a Haskell library for this should be very
approachable and clear to beginners, thus certain compromises were made. For
example, one cannot currently modify ManagerSettings of the default
manager because the library always uses the same implicit global manager for
simplicity and maximal connection sharing. There is a way to use your own
manager with different settings, but it requires a bit more typing.

“Type-safe” means that the library is protective and eliminates certain
classes of errors. For example, we have correct-by-construction URLs, it’s
guaranteed that the user does not send the request body when using methods
like GET or OPTIONS, and the amount of implicit assumptions is minimized by
making the user specify his/her intentions in an explicit form (for example,
it’s not possible to avoid specifying the body or method of a request).
Authentication methods that assume HTTPS force the user to use HTTPS at the
type level. The library also carefully hides underlying types from the
lower-level http-client package because those types are not safe enough
(for example Request is an instance of IsString and, if it’s malformed,
it will blow up at run-time).

“Expandable” refers to the ability to create new components for dealing with
HTTP without having to resort to ugly hacking. For example, it’s possible to
define your own HTTP methods, create new ways to construct the body of a
request, create new authorization options, perform a request in a different
way, and create your own methods to parse and represent a response. As a
user extends the library to satisfy his/her special needs, the new solutions
will work just like the built-ins. However, all of the common cases are also
covered by the library out-of-the-box.

“High-level” means that there are less details to worry about. The library
is a result of my experiences as a Haskell consultant. Working for several
clients, who had very different projects, showed me that the library should
adapt easily to any particular style of writing Haskell applications. For
example, some people prefer throwing exceptions, while others are concerned
with purity. Just define handleHttpException accordingly when making your
monad instance of MonadHttp and it will play together seamlessly. Finally,
the library cuts boilerplate down considerably, and helps you write concise,
easy to read, and maintainable code.

The library uses the following mature packages under the hood to guarantee
you the best experience:

It’s important to note that since we leverage well-known libraries that the
whole Haskell ecosystem uses, there is no risk in using Req. The machinery
for performing requests is the same as with http-conduit and Wreq. The
only difference is the API.

Motivation and Req vs other libraries

This section is my opinion and it contains criticisms of other well-known
libraries. If you’re user/fan of one of these libraries, please remember not
to react aggressively and respect the fact that I may have different views
on API design from yours.

I have spent time to write the library because sending HTTP requests is such
a common thing and still there is no high-level library for that in Haskell
that I could use with pleasure. I’ll explain why.

First of all, there is http-client and http-client-tls. They just work.
I have no issues with the libraries except that they are too low-level for
my taste. Indeed, even the docs say that they are low-level and “intended as
a base layer for more user-friendly packages”. This is exactly how I use
them in Req, as base level. Req is nothing but a different API to
http-client, so it only works because of the hard work put into
http-client.

http-conduit definitely has its place. For one thing it allows you to
stream request and response bodies in constant memory, what other library
allows you to do that? On the other hand if you take a look at
Network.HTTP.Simple, then although it’s said that it’s a “higher level
API”, it’s mostly the same as vanilla http-client in spirit/approach and
just adds conduit-powered functions to perform requests and allows to use
global implicit Manager (Req does the same). If I tried to frame what
exactly I don’t like about http-conduit in words, then it would be “the
way requests are constructed”. You set, set, set instead of being forced
to declare necessary bits and being allowed to declare optional bits in a
way that their combination is certainly valid. And you parse request from a
string without the protection of TH that otherwise saves the day as in
Yesod.

Then there is Wreq. wreqdoesn’t see much development
lately. wreq is by itself a weird
library, IMO. You have functions per method—not very good, as there may be
new methods, like PATCH which is not new but still missing (well you have
customMethod, but what is the point of having per-method functions if you
have a more general way to use any method? you should be able to just insert
methods in the “argument slot” of customMethod and end up with a more
general solution). Now every method function has a companion that takes
Options (like you have get and getWith). Why the duplication? Where is
generality and flexibility? This is not all though, because you cannot
really use get you see in the main module, because you want to have
connection sharing. Wreq’s author does not take the gift of automatic
connection re-use Manager from http-client provides, he invents the
whole new thing of “sessions”. Only inside a session your connections will
be shared and re-used. However with the session stuff you have yet another
set of per-method functions like get and getWith—these are different
ones, to be used with sessions! Now if you have a multi-threaded app, here
is a surprise for you: you can’t share connections between threads as
connections are shared only inside withSession friend and “session will no
longer be valid after that function returns”. There are valid uses for
sessions, but the point is that they are just too inconvenient for common
tasks.

It’s funny that one client I worked for had to have his own little wrapper
around http-client just because he could not possibly use wreq and
http-client and friends were too low-level. The previous paragraph is
extracted from a talk with a Haskell developer who works for that client. I
thought to myself “something is wrong with HTTP client libraries in Haskell
if they had to make a wrapper”.

What else? I used servant-client a couple of times but the amount of
boilerplate it requires is frightening. If you have several query
parameters, and you use just one of them, good luck passing lots of
Nothings.

Unsolved problems

AWS request signing is problematic because request body can be in the form
of an action to execute (and all that “popper” stuff for streaming), not
just a ByteString and so getting its digest (hash) is not trivial without
running the action and consuming body in its entirety before the request in
made. In Wreq the author chose to just use error when body is not a
(strict or lazy) ByteString. Maybe it’s OK for Wreq, but I don’t consider
this a proper solution for Req as we support full variety of body options.
For example, what if I want to upload 1 Gb file to S3? I want to stream it
in constant memory but at the same time I need to calculate its hash before
I start streaming. One solution to the problem seems to be in taking the
hash explicitly (as an argument of the hypothetical awsAuth) and making it
a responsibility of the user to calculate the hash correctly. I don’t like
this because it’s not user-friendly. So the question stays open, for now
there is no AWS signing functionality provided out-of-the-box. The best
solution for talking to AWS is the amazonka package so far.

Related packages

The following packages are designed to be used with Req:

req-conduit—support
for streaming request and response bodies in constant memory.

If you happen to have written a package that adds new features to Req,
please submit a PR to include it in this list.

Req 1.2.0

Req 1.1.0

Req 1.0.0

Added the reqBr function allowing to consume Response BodyReader
without using a pre-defined instance of HttpResponse, in a custom way.

Now streaming of response body does not happen until we’ve checked headers
and status code with httpConfigCheckResponse. It also doesn’t happen on
every retry. Streaming and obtaining of final response value happens only
once when we’re happy with everything.

Previously we first tried to consume and interpret response body before
checking status code and determining whether we should retry the request.
This was not good, because we could expect a JSON response but get a
response with status code 500, and then still we would try to parse it as
JSON first before letting httpConfigCheckResponse throw an exception.

The corrected behavior should also make retrying more efficient.

Changed signatures of several fields of HttpConfig:
httpConfigCheckResponse, httpConfigRetryPolicy, and
httpConfigRetryJudge in order to eliminate redundant IO and prevent
the possibility that these functions could start consuming BodyReader.

Removed the makeResponsePreview method from the HttpResponse type
class. Preview business is handled by the library automatically on a lower
level now. Users do not need to concern themselves with such stuff.

Changed the type signature of the getHttpResponse method of the
HttpResponse type class. Previously it left too much freedom (and
responsibility) to implementers of the method. In fact, we now limit what
getHttpResponse does to just consuming and interpreting Response BodyReader, so we can properly control details of connection
opening/closing etc., for the user.

Dropped support for GHC 7.8.

Minor documentation improvements.

Req 0.5.0

Changed the signature of the makeResponseBodyPreview from response -> IO ByteString to response -> ByteString.

Minor documentation improvements.

Req 0.4.0

Added the Req monad and runReq function to run it. This allows to use
req without defining new (orphan) instances.

Req 0.3.0

Removed the ReturnRequest HTTP response implementation as it was not
quite safe and was not going to work with retrying. Use req' instead for
“pure” testing.

Changed the type of httpConfigCheckResponse, so the second argument can
be any instance of HttpResponse.

Added built-in automatic retrying. See httpConfigRetryPolicy and
httpConfigRetryJudge in HttpConfig. The default configuration retries
5 times on request timeouts.

Added the makeResponseBodyPreview method to the HttpResponse type
class that allows to specify how to build a “preview” of response body for
inclusion into exceptions.

Improved wording in the documentation and README.md.

Req 0.2.0

Added support for multipart form data in the form of ReqBodyMultipart
body option and reqBodyMultipart helper function. This also required a
change in the type signature of getRequestContentType, which now takes
body, not Proxy body because we need to extract boundary from body
and put it into Content-Type header. This change, however, shouldn’t be
too dangerous for end-users.